No abstract

Tight sandstone has a complex pore-throat structure and usually has multiple immiscible displacement paths. Capillary number may control and affect this process, but its control mechanism is still unclear. Therefore, this work aims to study the control process of generalized capillary number on the immiscible displacement process of tight sandstone. Through nuclear magnetic resonance (NMR) online experiments, real-time monitoring of changes in remaining oil in different-radius pores was performed under the control of different generalized capillary numbers. Combined with the microscopic experimental results of a scanning electron microscope and thin-section and constant-speed mercury intrusion, the pore-throat distribution law was obtained, and a pore network model based on a real core was established. The results show that under the control of low capillary numbers, the immiscible displacement path shifts to large and medium pores, while under the control of medium and high capillary numbers, the displacement path shifts to medium and small pores. Moreover, the simulation results also showed interesting phenomena that were not realized in the experiment. When the number of capillaries increases to a certain value (Ca ≈ 6.734 × 10–2 in this study area), unfavorable viscous displacement will occur in tight sandstones, that is, the displacement efficiency will decrease rapidly.

The relationship between the electrical properties and relative permeability of tight sandstones with complex pore-throat structures is still unclear. In this study, a relationship model between the electrical parameters and pore-throat structure and the relative permeability of tight sandstone based on experimental data was established by combining theoretical derivation and experimental comparison. The model has typical three-terminal element characteristics. Porosity had little effect on relative permeability, whereas saturation index had a significant control effect on relative permeability. The relative permeability curve deduced based on the electrical parameters was quite different from the experimental fitting curve. Because irreducible water could conduct electricity but not flow, the relative permeability of the gas phase derived from the theory was higher than the experimental one, while the relative permeability of the water phase was lower. The isotonic point saturation of the phase permeability curve derived from the rock electrical parameter theory was larger than that obtained from the experiment. This research could help us obtain accurate relative permeability curves through electrical parameters and provide a basis for the fine evaluation of tight sandstone two-phase flow.